Car coolant overheating

Hello, I’m Pete from the Car Genius team at RepairPal. I’m sorry you’re dealing with this concern on your 2009 Chevrolet Tahoe. Based on the information provided it sounds like you had the cylinder head gaskets replaced as well as the cylinder heads resurfaced and after you are experiencing after a short run time that the engine is overheating and boiling over coolant with or without your thermostat being installed. I recommend to first check and make sure your engine drive belt is routed properly to ensure that the water pump is rotating in the right direction. Next I recommend making sure you have all of the air bled from the cooling system as trapped air can cause this concern. If these are checked into and found to be o.k. then I recommend having what is called a "block test" or "combustion leak test" performed to see if any exhaust is getting into your cooling system. This test involves using a simple tester and fluid that seats on your radiator reservoir spout, the engine is then run to see if any bubbling occurs in the tester and to see if the blue fluid turns to a greenish or yellowish color. If color change is found this indicates that exhaust is leaking past your new cylinder head gaskets into your cooling system, or, if you know the cylinder heads are good with no cracks or damage and are even, this can indicate a concern with one of the cylinder walls or the engine block itself being cracked. If it is found that exhaust is entering the cooling system further testing and inspection will be needed to pinpoint the failure so it can then be repaired properly from there. I wish I had some better news to give you on this. If you need assistance connecting with one of our RepairPal Certified shops, more advice, or have any further questions about your vehicle, we’re here to help. Just give us a call at 877-913-1554 where we will be happy to assist!